Control device for hybrid four-wheel-drive vehicle and hybrid four-wheel-drive vehicle

ABSTRACT

A control device controls a hybrid-four-wheel-driven vehicle wherein one of the front-wheel pair and the rear-wheel pair is an engine-driven-wheel pair which is driven by an engine, and the other pair is an electric-motor-driven-wheel pair which is driven by an electric motor. In the event of slipping of the engine-driven wheels, the system increases the engine output so as to increase the driving force of the electric-motor-driven wheels for increasing the total driving force made up of the engine-driven-wheel driving force and the electric-motor-driven-wheel driving force, thereby improving acceleration performance, unlike conventional techniques. This provides a control device for suppressing deterioration in acceleration performance of the vehicle in a case of slipping of engine-driven wheels during acceleration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control device for a hybridfour-wheel-drive vehicle wherein the front or rear wheels are driven byan engine, and the others are driven by an electric motor.

2. Description of the Related Art

As an example of hybrid four-wheel-drive vehicles, a hybridfour-wheel-drive vehicle wherein the front wheels are driven by anengine, and the rear wheels are driven by an electric motor is known, asdisclosed in Japanese Unexamined Patent Application Publication No.2001-63392, paragraph 0004, for example (which will be referred to as“Patent Document 1” hereafter). With the four-wheel-drive vehicledisclosed in this Patent Document 1, in the event that the front wheelsslip or spin (which will be generically referred to as “slipping”hereafter) during acceleration, the engine drives a generator serving asa power supply for an electric motor for driving the rear wheel. Withsuch a configuration, a part of the engine output is used for drivingthe generator for supplying electric power to the electric motor,thereby reducing the driving force of the front wheels so as to preventslipping of the front wheels. Furthermore, even in the event that suchoperation is insufficient for suppressing slipping of the front wheel,the system reduces the engine output so as to suppress slipping of thefront wheels in a sure manner.

However, with the control method disclosed in Patent Document 1, in theevent that the engine-driven wheels slip during acceleration, the engineoutput is reduced so as to suppress slipping, leading to reducedrotational speed of the generator driven by the engine, and reducedtorque for driving the generator, resulting in reduced generatedelectric power. This leads to reduced electric-current supply to theelectric motor, resulting in insufficient torque of the electric motor.Furthermore, in the event that the induced voltage of the electric motorexceeds the voltage generated by the generator, the generator cannotsupply current to the electric motor, may lead to a problem that theelectric motor cannot be driven in such cases. With the aforementionedconfiguration further including a mechanism wherein an electricitystorage device such as a storage battery or the like supplies electricpower to the electric motor in such a case, the electric motor generatessufficient torque even in such a case. However, with such aconfiguration, continuous driving may reduce the electricity stored inthe electricity storage device, may lead to a problem that the electricmotor cannot be driven in such a case. As described above, the controlmethods according to the conventional technique have a problem that inthe event that the engine-driven wheels slip during acceleration, theelectric-motor-driven wheels may not be driven with sufficient drivingforce, leading to reduction of the acceleration performance of thevehicle.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a control method forimproving the acceleration performance of the vehicle even in a case ofthe engine-driven wheels slipping during acceleration.

In order to solve the above-described problems, a control device forcontrolling a hybrid-four-wheel-driven vehicle according to an aspect ofthe present invention, wherein one of the front-wheel pair and therear-wheel pair is an engine-driven-wheel pair which is driven by anengine, and the other pair is an electric-motor-driven-wheel pair whichis driven by an electric motor connected to a generator driven by theengine, comprises: slipping detecting means for detecting slipping ofthe engine-driven wheels; and output control means for increasing theoutput of the engine corresponding to the increased output of theelectric motor.

That is to say, the present invention has been made based upon the factthat in a case of slipping of the engine-driven wheels duringacceleration, the increased driving force of the electric-motor-drivenwheels by increasing engine output increases the total driving force ofthe engine-driven-wheel driving force and theelectric-motor-driven-wheel driving force, unlike conventionaltechniques. In this case, while slipping of the engine-driven wheelscannot be suppressed, in general, the increase of the effectiveelectric-motor-driven-wheel driving force can be adjusted to be greaterthan the decrease of the effective engine-driven-wheel driving force.Thus, the present invention improves acceleration performance.

With the above-described control device, the slipping detecting meansfor the engine-driven wheels may be started at the time of receiving arequest for acceleration of the vehicle. The reason is that in general,slipping occurs during acceleration. Note that the slipping detectingmeans may detect slipping in the event that the rotational speed of theengine-driven wheels exceeds the rotational speed of theelectric-motor-driven wheels. Furthermore, the slipping detecting meansmay detect slipping in the event that the speed of the engine-drivenwheels exceeds the driving speed of the vehicle. Furthermore, theslipping detecting means may detect slipping in the event that theslippage, which is the difference in rotational speed between theengine-driven wheels and the electric-motor-driven wheels, divided bythe driving speed of the vehicle, is equal to or greater than apredetermined value.

In the above-described configuration, the output control means mayfurther comprise: means for computing the present electric-motor outputbased upon an input current and a field-coil current of the electricmotor; means for computing target acceleration driving forcecorresponding to an input acceleration request; means for obtainingtarget electric-motor output based upon the present electric-motoroutput and the target acceleration driving force; means for obtainingtarget engine output required for achieving the target electric-motoroutput; and means for controlling output of the engine and output of theelectric motor according to the target engine output and the targetelectric-motor output.

Furthermore, the output control means may further comprise:effective-driving-force history computation means for obtaining historydata of the effective driving force of the electric-motor driving wheelscorresponding to the last slippage data for a predetermined past period;and maximum-effective-driving-force computation means for computing themaximum value of the effective driving force of theelectric-motor-driven wheels based upon the history data, with theoutput of the electric motor being increased in a range determined bythe maximum value of the effective driving force of theelectric-motor-driven wheels.

Furthermore, a control device for controlling a hybrid-four-wheel-drivenvehicle according to another aspect of the present invention, whereinone of the front-wheel pair and the rear-wheel pair isengine-driven-wheel pair which is driven by an engine, and the otherpair is an electric-motor-driven-wheel pair which is driven by anelectric motor, comprises: slipping detecting means for detectingslipping of the engine-driven wheels; first output control means forreducing the output of the engine and reducing the output of theelectric motor corresponding to the reduction of engine output when theslipping detecting means detect slipping; second output control meansfor increasing the output of the electric motor and increasing theoutput of the engine corresponding to the increase of electric-motoroutput when the slipping detecting means detect slipping; and switchingmeans for making switching between the first output control means andthe second output control means. Note that the first output controlmeans comprise control method according to conventional techniquesdisclosed in the Patent Document 1, wherein the system gives priority tosuppression of excessive slipping of the engine-driven wheels duringacceleration.

With such a configuration, the system switches the selected control modeto the second output control means for giving priority to output of theelectric motor under conditions such as driving on an icy uphill slope,and accordingly, the driving force of the electric-motor-driven wheelsis increased, thereby improving acceleration performance. Subsequently,the system switches the selected control mode back to the first outputcontrol means for giving priority to suppression of excessive slippingof the engine-driven wheels, thereby suppressing deterioration in thelifespan of the electric motor due to excessive use thereof, and therebysuppressing deterioration in driving performance of the vehicle.

In this case, the switching means for making switching between the firstand second output control means may comprise a switch, and furthermore,switching therebetween may be automatically made. For example, anarrangement may be made wherein the switching means predict totaleffective driving forces according to the first output control means andthe second output control means, each of which include the effectivedriving force of the engine-driven wheels and the effective drivingforce of the electric-motor-driven wheels, and the switching meansswitch the presently-selected output control method to the outputcontrol method corresponding to the one of the total effective drivingforces predicted to have a greater value. Furthermore, the controldevice further comprises steering-amount detecting means for detectingthe steering amount of the vehicle, and in the event that the steeringamount detected by the steering-amount detecting means is equal to orgreater than a predetermined value, the switching means select the firstcontrol means. With the vehicle having a configuration wherein thesteering wheels is driven by the engine controlled according to thesecond output control means, steering of the engine-driven wheelsgenerates small lateral force of the wheels in a situation whereinacceleration slipping occurs, often leading to a problem ofunder-steering. Accordingly, an arrangement may be made wherein at thetime of steering for turning a corner while making acceleration, thesystem switches the selected output control means to the first outputcontrol means. With such a configuration, the vehicle generates yawmoment more quickly, thereby improving turning-round performance of thevehicle. On the other hand, at the time of driving of the vehicle at alow speed, in many cases, the great yaw moment is not required.Accordingly, an arrangement may be made wherein the system makes forcedswitching from the second control method to the first control methodaccording to detection of steering in a case of the present vehiclespeed exceeding the first vehicle-speed threshold (e.g., 8 km/h). Withsuch a configuration, the vehicle maintains the great rear-wheel drivingforce at a low speed even in a case of the user steering the vehicle,thereby maintaining acceleration performance of the vehicle.

The present invention thus suppresses deterioration in accelerationperformance of the vehicle in a case of slipping of the engine-drivenwheels during acceleration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall configuration diagram which shows a hybridfour-wheel-drive vehicle according to an embodiment of the presentinvention;

FIG. 2 is a functional configuration diagram which shows a controlleraccording to an embodiment of the present invention;

FIGS. 3A and 3B illustrate the relation between the voltage generated bya generator and the torque of the generator, and the relation betweenthe generated voltage and the generated current, with the rotationalspeed as a parameter;

FIGS. 4A and 4B illustrate the relation between the voltage generated bythe generator and the torque of the generator, and the relation betweenthe generated voltage and the generated current, with the field-coilcurrent as a parameter;

FIG. 5 is a chart for describing the relation between the slippage andthe effective driving force;

FIG. 6 is a chart which shows the transmission properties of a torqueconverter;

FIG. 7 shows charts for describing the relation between: the slippage ofthe front and rear wheels; and the effective driving force thereof,before and after control processing with a first control methodaccording to the present invention;

FIG. 8 shows charts for describing the relation between: the slippage ofthe front and rear wheels; and the effective driving force thereof,before and after control processing with a second control methodaccording to the present invention;

FIG. 9 is a flowchart for describing the processing performed byelectric-motor output control means according to an embodiment of thepresent invention;

FIG. 10 is a flowchart for making detailed description regarding theprocessing performed in Step S1 shown in FIG. 9;

FIG. 11 is a flowchart for making detailed description regarding theprocessing performed in Step S8 shown in FIG. 9;

FIG. 12 is a flowchart for making detailed description regarding theprocessing performed in Step S10 shown in FIG. 9;

FIG. 13 is a flowchart for making detailed description regarding theprocessing performed in Step S9 shown in FIG. 9;

FIGS. 14A through 14I show time charts for describing change in thedriving state of each component of the vehicle according to the secondcontrol method of the present invention;

FIGS. 15A and 15B are diagrams which show configuration of a switch formaking switching between the first and second control methods accordingto the present invention; and

FIG. 16 is a flowchart for describing automatic switching processing formaking switching between the first and second control methods accordingto the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will be made below regarding a hybrid four-wheel-drivevehicle and a control device thereof according to an embodiment of thepresent invention with reference to the drawings. Note that whiledescription will be made regarding the present embodiment forsimplification by way of an arrangement example wherein the vehicle hasengine-driven wheels 9 driven by an engine 1, serving as the frontwheels, and electric-motor-driven wheels 8 driven by an electric motor5, serving as the rear wheels, it is needless to say that the presentinvention may be applied to an arrangement wherein the vehicle has theelectric-motor-driven wheels 8 driven by the electric motor 5, servingas the front wheels, and the engine-driven wheels 9 driven by the engine1, serving as the rear wheels, as well.

As shown in FIG. 1, a driving device of a hybrid four-wheel-drivevehicle according to the present embodiment includes the engine 1 andthe electric motor 5. The output from the engine 1 is transmitted to theengine-driven wheels 9 through a transmission 7 including a torqueconverter. The output from the electric motor 5 is transmitted to theelectric-motor-driven wheels 8 through a differential gear 6 including aclutch. A generator 2 is electrically connected to the electric motor 5through a power line 10.

The generator 2 is driven by the engine 1 through an unshownaccelerating pulley mechanism. In the event that the voltage generatedby the generator is less than the voltage supplied from an unshownengine-starting battery, the battery supplies current to a field coil 2a of the generator 2. On the other hand, in the event that the voltagegenerated by the generator 2 exceeds the battery voltage, a part of thecurrent generated by the generator 2 is supplied to the field coil 2 a.With the present embodiment, the electric power generated by thegenerator 2 is controlled by a controller 4 controlling the currentsupplied to the field coil 2 a with the PWM (Pulse Width Modulation)method. As described above, the generated electric power is controlledby the controller 4 adjusting the field electric current. As a result,the driving force of the electric-motor-driven wheel 8 driven by theelectric motor 5 is controlled by the controller 4.

On the other hand, the electric motor 5 is a DC electric motor driven byelectric current supplied from the generator 2. The system controls thetorque coefficient of the electric motor 5 by adjusting the fieldcurrent supplied to the field coil 5 a. This enables the electric motor5 to rotate at a high rotational speed, unlike a permanent magnet DCelectric motor. With the present embodiment, the controller 4 adjuststhe voltage supplied from an unshown battery with the PWM method so asto control the current supplied to the field coil 5 a. The output shaftof the electric motor 5 is connected to the electric-motor-driven wheels8 through the differential gear 6. The current supplied to the electricmotor 5 is detected by a current sensor 14 provided at a predeterminedportion on the power line 10. As described above, the control deviceincludes the field coil 2 a of the generator 2 and the field coil 5 a ofthe electric motor 5 serving as the output control means for theelectric motor 5.

On the other hand, with the control device according to the presentembodiment, the intake of the engine 1 includes an electronic controlthrottle 1 a for adjusting suction air flow, serving as means forcontrolling the output of the engine 1. The controller 4 adjusts theelectronic control throttle 1 a so as to control the output of theengine 1. Furthermore, the controller 4 monitors the engine rotationalspeed through a rotational speed sensor 1 b mounted on the engine 1.

Furthermore, the control device according to the present embodimentincludes wheel-speed sensors 11, each of which are mounted on thecorresponding wheel, serving as wheel-speed detecting means fordetecting the rotational speed of the engine-driven wheels 9 and theelectric-motor-driven wheels 8, as well as serving as wheel-speeddetecting means for the ABS (Antilock Brake System). The data of thedetected rotational speed of each driven wheel is transmitted to thecontroller 4. Furthermore, the control device according to the presentembodiment includes an accelerator-pedal sensor 13 for detecting thestepping amount of the accelerator, serving as acceleration requestdetecting means for detecting a request for acceleration of the vehicle,and the detected signals are transmitted to the controller 4.Furthermore, the controller 4 receives signals which indicate the stateof a switch 12 for selecting a control method from a first controlmethod and a second control method described later in a case of slippingof the engine-driven wheels 9 serving as the front wheels duringacceleration.

In FIG. 2, a functional block diagram of a control software configuringthe controller 4 is shown. In the drawing, acceleration requestdetecting means 31 detect the request for acceleration of the vehiclebased upon the stepping amount of the accelerator pedal from the user.An engine-driven wheel-speed detecting means 32 detect the speed of thefront wheels using the wheel-speed sensors 11 mounted on the frontwheels. An electric-motor-driven wheel-speed detecting means 33 detectthe speed of the rear wheels using the wheel-speed sensors 11 mounted onthe rear wheels. Engine output control means 34 calculate the rotationalspeed and the torque of the engine, and adjust the electronic controlthrottle such that the actual rotational speed and the torque thereofmatch the target ones, whereby the output of the engine is controlled.Electric-motor output control means 35 receive the acceleration requestoutput from the acceleration request detecting means 31, the data of thefront wheel speed output from the engine-driven wheel-speed detectingmeans 32, and the data of the rear wheel speed output from theelectric-motor-driven wheel-speed detecting means 33, following whichthe system determines the target output of the electric motor 5 basedupon the aforementioned received information. Then, the electric motoroutput control means 35 compute the target rotational speed and thetarget torque of the engine 1, which drives the generator 2, forcontrolling the electric power generated by the generator 2 such thatthe actual output of the electric motor matches the target one. Then,the electric motor output control means 35 adjust the field-coil currentof the generator 2 so as to control the generated electric power, aswell as transmitting the data of the target engine rotational speed andthe target engine torque thus obtained to the engine output controlmeans 34.

FIGS. 3A and 3B show properties of the generator with the field-coilcurrent of the generator controlled with the PWM ratio of 100% (i.e.,with the continuous current mode). FIG. 3A shows the relation betweenthe torque of the generator and the generated voltage, and FIG. 3B showsthe relation between the generated current and the generated voltage.Note that these properties are shown with the rotational speed of thegenerator as a parameter, and the solid lines A, B, and C, represent theproperties of the generator with the rotational speed of the generatoras a parameter, wherein the rotational speed increases in that order ofA, B, and C. As can be understood from the properties represented by thesolid lines A, B, and C, the greater the rotational speed is, thegreater the generated electric power is, and accordingly, the generatorhas the property wherein the generated voltage increases in a case ofincreasing the rotational speed while maintaining the generated current.The term “generated voltage”, which is represented by the horizontalaxis in FIGS. 3A and 3B, means the voltage applied to the load connectedto the generator. With the present embodiment, the power line 14 and theelectric motor 5 serve as the load. As can be understood from comparisonbetween the voltage drop due to the power line 14 and that due to theelectric motor 5, the voltage drop due to the power line 14 is extremelysmall. Accordingly, let us consider the voltage drop due to the electricmotor 5 alone. The voltage drop due to the electric motor 5 is mainlydue to the voltage occurring due to the counter electromotive force ofthe electric motor 5. Driving of the electric motor 5 generates thecounter-electromotive force corresponding to the rotational speed andthe torque of the electric motor, leading to the voltage (which will bereferred to as “counter-electromotive voltage” hereafter) inverse of thevoltage generated by the generator. The following Expression (1)represents the relation between the rotational speed ωmot of theelectric motor and the counter-electromotive voltage Eemf occurring dueto the counter electromotive force thereof. Here, Ke represents aproportional constant.Eemf=Ke·ωmot  (1)

It is needless to say that current is supplied from the generator 2 tothe electric motor 5 in the event that the counter-electromotive voltageis less than the voltage generated by the generator. However, in theevent that the counter-electromotive voltage becomes the same as thevoltage generated by the generator, current is not supplied from thegenerator 2 to the electric motor 5, leading to output of zero. Forexample, in a case of driving the generator 2 at the rotational speed B,in the event that the counter-electromotive voltage is equal to orgreater than VmaxB (see the solid line B in FIG. 3B), current is notsupplied from the generator to the electric motor 5. Accordingly, thegenerator 2 needs to be driven at a rotational speed increasedcorresponding to the counter-electromotive voltage increased due to theincreased rotational speed of the electric motor 5 for allowing themotor 5 to generate torque. Even in a case wherein current is suppliedto the electric motor 5, a large amount of current is required forincreasing the torque of the electric motor 5, and accordingly, there isthe need to increase the rotational speed of the generator 2.Conversely, in the event that excessive current is generated due to theincreased rotational speed of the generator 2, the system can reduce thegenerated current by reducing the field-coil current of the generator 2.As an example, FIGS. 4A and 4B show properties of electric-powergeneration with the field-coil current controlled by the controller 4 asa parameter while maintaining the rotational speed of the generator 2 ofC. Specifically, these properties are shown with the field-coil currentcontrolled by the controller 4 as a parameter, which is represented bythe PWM ratio. Note that the PWM ratio of 100% represents the continuouscurrent mode, and the PWM ratio of 0% represents the current-off mode.

Detailed description will be made below regarding to a configuration ofthe control device of the hybrid-four-wheel-drive vehicle according tothe present embodiment having such a configuration, as well as regardingthe operation thereof. First, let us consider a case wherein the outputof the engine 1 is increased so as to accelerate the vehicle on a lowfriction road such as an icy road. In this case, excessive torquetransmitted from the engine 1 causes slipping of the engine-drivenwheels 9 serving as the front wheels. The known relation between theslippage and the effective driving force is shown in FIG. 5. In FIG. 5,the horizontal axis represents the slippage, and the vertical axisrepresents the effective driving force of the driving wheels. Note thatthe term “slippage” used here means a dimensionless value represented bythe following Expression (2), for example. On the other hand, the term“effective driving force” used here means a driving force obtained bysubtracting the torque used for acceleration of the driving wheels fromthe driving force input to the driving wheels, i.e., a driving forcewhich contributes acceleration of the vehicle as to the road.Slippage=((front-wheel speed)−(rear-wheel speed))/(vehiclespeed)=((driven-wheel speed)−(vehicle speed))/(vehicle speed)  (2)

As shown in FIG. 5, this relation has a slippage point (e.g., 5% to 20%)corresponding to the maximum effective driving force. From theaforementioned slippage point, the greater the slippage is, the smallerthe effective driving force is while approaching a certain effectivedriving force which is an asymptotic value denoted by referencecharacter B. Accordingly, in general, the slippage of the front wheels,which are driven by an engine, is preferably reduced to around the peakdenoted by reference character A for effectively using the effectivedriving force for accelerating the vehicle.

With the vehicle having a configuration wherein the torque of the engine1 is transmitted to the front wheels through a torque converter, thesystem reduces the rotational speed of the engine so as to reduce thedriving torque of the wheels for reducing the slippage of the frontwheels, giving consideration to the driving-force transmissionproperties of the torque converter. FIG. 6 shows the properties of thetorque converter. In the drawing, the horizontal axis represents thespeed ratio of the input/output shafts of the torque converter (=(speedof the output shaft)/(speed of the input shaft)), and the vertical axisrepresents the torque ratio of the input/output shafts (=(torque of theoutput shaft)/(torque of the input shaft)). As can be clearly understoodfrom FIG. 6, the smaller the speed ratio is, the greater the torqueratio is. Note that the input shaft is directly connected to the engine,and the output shaft is connected to the wheels through a reductionmechanism. Accordingly, it is effective for reducing the driving torqueof the wheels to reduce the engine output so as to reduce the rotationalspeed thereof.

Note that with the hybrid-four-wheel-drive vehicles, in some cases, theoperation wherein the slippage of the front wheels is reduced so as toachieve the optimum effective driving force thereof does not maintainthe optimum acceleration performance of the vehicle. The reason is thatreduction of the rotational speed of the engine 1 leads to reduction ofthe electric power generated by the generator, resulting in reducedoutput of the electric motor 5 for driving the rear wheels. Now,description will be made regarding this phenomenon with reference toFIG. 7. FIG. 7 is a diagram for describing the relation between theslippage and the effective driving force for the front wheels and therear wheels in the initial stage and the stage where the system adjuststhe engine output according to a first method of the present invention.The left side in FIG. 7 shows the relation between the effective drivingforce and the slippage for the front and rear wheels in the initialstage. As can be understood from the operating point denoted by opencircle in the drawing, the slippage of the front wheels is great, andthe effective driving force thereof is small as compared with themaximum value. On the other hand, the right side in FIG. 7 shows therelation between the slippage and the effective driving force for thefront and rear wheels in the stage where the system reduces the outputof the engine 1 such that the effective driving force of the frontwheels exhibits the maximum value. As can be understood from theoperating points denoted by the open circles in the drawing, while theeffective driving force of the front wheels increases, the effectivedriving force of the rear wheels is reduced due to reduction of theoutput of the electric motor 5. Note that reduction of the effectiverear-wheel driving force is greater than the increase of the effectivefront-wheel driving force. This leads to reduction of the totaleffective driving force of the vehicle, which is obtained by making thesum of the effective driving forces of the front wheels and the rearwheels.

On the other hand, with a second control method according to the presentinvention, in such an initial stage described above, the systemincreases the engine output so as to increase the output of the electricmotor 5 for increasing the driving force of the rear wheels driven bythe electric motor, unlike the first control method described above. Insome cases, the second control method achieves the increased totaleffective driving force of the vehicle, thereby maintaining the optimumacceleration performance thereof, as compared with the first controlmethod. Now, description will be made regarding this phenomenon withreference to FIG. 8. FIG. 8 is a diagram for describing the relationbetween the slippage and the effective driving force for the frontwheels and the rear wheels in the initial stage and the stage where thesystem adjusts the engine output according to a second method of thepresent invention. The left side in FIG. 8 shows this relation in theinitial stage, wherein the slippage of the front wheels is greater thanthat of the rear wheels. On the other hand, the right side in FIG. 8shows this relation in the stage where the system increases the engineoutput so as to increase the output of the electric motor according tothe second control method of the present invention. In this case, whilethe effective driving force of the front wheels is reduced, the totaleffective driving force of the vehicle increases since the increase ofthe effective driving force of the rear wheels is greater than thereduction of the effective driving force of the front wheels, therebyimproving the acceleration performance of the vehicle.

Furthermore, in a case of slipping of the engine-driven wheels, thesystem according to the present embodiment selects a suitable controlmethod corresponding to a situation, from the first control methodwherein the system reduces the engine output so as to suppress slippingthereof for accelerating the vehicle, and the second control methodwherein the system increases the engine output, unlike the first controlmethod, so as to increase the output of the electric motor foraccelerating the vehicle. Specific description thereof will be madebelow.

FIGS. 9 through 13 are flowcharts for describing control processingperformed by output control means formed of the electric-motor outputcontrol means 35 and the engine output control means. FIG. 9 shows thecontrol processing performed by the electric-motor output control means35, which is the most essential part of the present embodiment. In StepS1, the system determines the presence or absence of the accelerationrequest, and calculates the acceleration request value. In Step S2, inthe event that the system determines the presence of the accelerationrequest, the flow proceeds to Step S3, where the system performsfollowing processing, otherwise, the processing ends. In Step S3, thesystem calculates the slippage based upon the front-wheel speed and therear-wheel speed according to Expression (2) for detecting the slippageof the front wheels. In Step S4, the slippage is compared to apredetermined value (e.g., 20%) so as to make determination whether ornot slipping of the front wheels occurs. Note that while description hasbeen made regarding an arrangement wherein the system detects theslipping of the front wheels in the event that the slippage, which isobtained by dividing the difference in the rotational speed between theengine-driven wheels and the electric-motor-driven wheels by the drivingspeed of the vehicle, exceeds a predetermined value, an arrangement maybe made wherein the system detects the slipping thereof in the eventthat the speed of the engine-driven wheels exceeds the speed of thevehicle, and furthermore, an arrangement may be made wherein the systemdetects the slipping thereof in the event that the rotational speed ofthe engine-driven wheels exceeds the rotational speed of theelectric-motor-driven wheels.

In the determining Step, i.e., Step S4, in the event that the systemdetects the slipping of the front wheels, the flow proceeds to Step S5,otherwise, the processing ends. In Step S5, determination is madewhether the system performs control processing according to the firstcontrol method wherein the engine output is reduced for suppressing theslipping of the front wheels, or according to the second method whereinthe engine output is increased for increasing the driving force of therear wheels. Note that the system may determine the control methodaccording to the signals received from the switch 12 for the userswitching the control method, or the system may be automaticallydetermined the control method based upon the wheel speed and thestepping amount of the accelerator pedal from the user, as describedlater. For example, in the event that the user selects the first controlmethod through the switch 12, the system sets the flag stored within thecontroller, CntrlFlag, to 1. On the other hand, in the event that theuser selects the second control method through the switch 12, the systemsets the flag CntrlFlag to 2.

In Step S6, the system checks the flag which indicates the determinationresults obtained in Step S5, and in the event that the flag CntrlFlagmatches “1”, the flow proceeds to Step S7, where the system performscontrol processing according to the first control method. On the otherhand, in the event that the flag CntrlFlag matches “2”, the flowproceeds to Step S8, where the system performs control processingaccording to the second control method. In Step S7, the system performscontrol processing according to the first control method, wherein thesystem reduces the output of the engine so as to suppress the slippingof the front wheels, and accordingly, the output of the generator isreduced corresponding to the reduction of the engine output, whereby theprocessing ends.

The control processing according to the second control method isperformed in Steps S8 through S11. First, in Step S8, the systemcalculates the present output torque of the electric motor. Thecomputing method is shown in FIG. 11. Next, in Step S9, the systemdetermines the target electric-motor torque (output) of the electricmotor based upon the acceleration request value for the vehicle; andfurther determines the target engine rotations, the target enginetorque, and the target field-coil current of the generator, required foroutputting the target electric-motor torque. The detailed computingmethod is shown in FIG. 13. In Step S10, the engine output control means34 perform output control of the engine based upon the target enginerotations and the target engine torque. Subsequently, in Step S11, theelectric-motor output control means 35 control the field-coil current ofthe generator based upon the target electric-motor torque, whereby theprocessing ends.

The system performs the processing described above, whereby theprocessing shown on the right side in FIG. 8 is performed, therebysuppressing deterioration in the acceleration performance of the vehiclein a case of slipping of the engine-driven wheels during acceleration ofthe vehicle.

Detailed description will be made below regarding the principal stepswith reference to FIGS. 10 through 13. FIG. 10 is a flowchart for makingdetailed description regarding the method for determining the presenceor absence of the acceleration request, which is performed in Step S1.First, in Step S61, the system reads out and stores the signal Th[−1]from the accelerator-pedal sensor 13. Next, in Step S62, the systemreads out the next signal Th[0] from the accelerator-pedal sensor 13 inthe same way. Then, in Step S63, the system calculates and stores thevalue ΔTh wherein the new value Th[0] from the accelerator-pedal sensor13 is subtracted from the old value Th[−1] thereof. In Step S64, in theevent that ΔTh is a positive value, the system determines that the userhas stepped the accelerator pedal, and the flow proceeds to Step S65,wherein the system sets the acceleration request flag ACD to “1”,whereby the step ends. On the other hand, in Step S64, in the event thatΔTh is a negative value, the system determines that the accelerationrequest has not made, the flow proceeds to Step S66, where the systemsets the acceleration request flag ACD to “0”, whereby the processingends.

FIG. 11 is a flowchart for making detailed description regarding theprocessing for computing the present motor output torque, which isperformed in Step S8. In Step S71, the current sensor 14 detects thecurrent supplied to the electric motor, i.e., the generated current Ia.In Step S72, the system estimates the current Ifm supplied to the fieldcoil of the electric motor based upon the PWM ratio controlled by thecontroller 4. In Step S73, the system calculates the electric-motortorque Tm based upon the aforementioned information, whereby the stepends. Here, the electric-motor torque Tm is determined by the product ofthe generated current Ia and the electric-motor torque constant Kt. Notethat Kt is determined dependent upon the field-coil current, andaccordingly, with the present embodiment, the system stores the relationbetween the torque constant Kt and the PWM ratio in the form of a tablebeforehand, and the torque constant Kt is determined for each case bysearching the table.

FIG. 12 is a flowchart for making detailed description regarding theprocessing for controlling the engine output performed in Step S10. InStep S81, the system calculates the engine rotational speed Ne basedupon the signals received from the engine-rotation sensor 1 b. In StepS82, the system detects the throttle position TVO using a throttleposition sensor which can be electronically controlled. Subsequently, inStep S83, the system calculates the engine torque Te based upon theengine rotational speed Ne and the throttle position TVO using the dataof the relation between the engine rotational speed, the aperture of thethrottle valve, and the engine torque, which has been stored beforehand.Subsequently, in Step S84, the system calculates the difference ΔNewherein the target engine rotational speed Netrgt is subtracted from theengine rotational speed Ne, as well as calculating ΔTe which is thedifference between the engine output torque Te and the target enginetorque Tetgt. Note that the target engine rotational speed Netrgt andthe target engine torque Tetgt are obtained in the processing shown inFIG. 13, which will be described later. Subsequently, in Step S85, inthe event that ΔNe or ΔTe is zero or more, the flow proceeds to StepS86, where the throttle is reduced. Otherwise, the flow proceeds to StepS87 where the throttle is increased, following which the processingends.

FIG. 13 is a flowchart for making detailed description regarding theprocessing for controlling the output of the electric motor performed inStep S9. In Step S91, the system computes the target electric-motortorque Tmtgt. The target electric-motor torque Tmtgt is calculated asfollows. That is to say, the change in the stepping amount of theaccelerator pedal, ΔTh, indicating the acceleration request value whichhas been calculated, is multiplied by a constant C1, and the sum of theproduct thus obtained and the present motor torque Tm is made, wherebythe sum thus obtained is determined as the target electric-motor torqueTmtgt. Note that the value C1 used here is not restricted to a constant,rather, an arrangement may be made wherein C1 is determinedcorresponding to ΔTh such that the change in the target electric-motortorque is suppressed for a small stepping amount of the acceleratorpedal, thereby enabling smooth acceleration of the vehicle.

On the other hand, the output of the electric motor is dependent uponfour parameters of: (1) the generated current Ia; (2) the field-coilcurrent Ifm of the electric motor; (3) rotational speed Ne of the engineproportional to the rotational speed Na of the generator; and (4) thefield-coil current Ifa of the generator. In this case, there aremultiple parameters for achieving the desired target electric-motortorque, and accordingly, with the present embodiment, the system selectsa combination of the parameters such that the present operation state ofthe engine exhibits as small a change as possible. While this leads toincreased calculation amounts, the system has the first advantage ofsuppressing the great change in the engine speed, thereby enablingacceleration of the vehicle without unpleasant sensation of thepassengers. Furthermore, the system has the second advantage ofimmediately achieving the target electric-motor output torque, sincethere is no need to greatly change the output of the engine which has aslow response as compared with the generator and the electric motor.

Description will be made regarding the processing procedure in Steps S92through S911, wherein the output control conditions are determined suchthat the engine operation state exhibits as small change from thepresent engine operation state as possible for outputting the targetelectric-motor torque. In FIG. 13, in Step S92, the system set a counteri (i denotes an integer) for counting the number wherein the loopprocessing has been performed, more specifically, the system incrementsthe counter by 1 for each loop processing. In Step S93, the PWM ratio ofthe field-coil current Ifm is divided into an n (n represents aninteger) number of PWM ratio units from 0% up to 100%, and thefield-coil current corresponding to the i-numbered PWM ratio unit is setto the temporary target motor-field-coil current Ifmtgt. In Step S94,the target electric-motor torque Tmtgt, which has been obtained in StepS91, is divided by the torque constant Kt for calculating the targetcurrent Iatgt. The torque constant Kt is dependent upon theelectric-motor-field-coil current Ifmtgt, and accordingly, the systemdetermines the torque constant Kt using a table wherein the relationtherebetween has been stored beforehand. In Step S95, the systemdetermines the target motor rotational speed Nmtgt, independent of theaforementioned processing. Here, the system may determine the targetmotor rotational speed Nmtgt to be the present motor rotational speed,for example. Furthermore, the system preferably determines the targetmotor rotational speed Nmtgt to be a value greater than the presentmotor rotational speed, corresponding to the aperture of the acceleratorpedal for correcting the response of the accelerator pedal. In Step S96,the system computes the product of the target motor rotational speedNmtgt and the torque constant Kt, whereby the target generated voltageEatgt is determined to be the product thus obtained.

In Step S97, the system calculates the engine rotational speed Ne, thegenerator field-coil current Ifmtgt, and the generator torque Ta, forachieving both the target generated voltage Eatgt and the targetgenerated current Iatgt. Note that the system prepares a map includingthe relation between: the generated voltage; the generated current; thefield-coil current; and the generator torque, for each engine rotationNe, (e.g., by steps of 100 rotations/minute), for performing theaforementioned computation processing. Specifically, the system searchesthe map for the values corresponding to the target generated voltageEatgt and the target current Iatgt, whereby the engine rotational speedNe, the generator field-coil current Ifmtgt, and the generator torqueTa, are obtained corresponding to the target generated voltage Eatgt andthe target current Iatgt. In the event that the system has picked up themultiple values of the engine rotational speed, the system selects theengine rotational speed closest to the present engine rotational speedas the target engine rotational speed Netgt, and selects thecorresponding generator field-coil current as the target generatorfield-coil current Ifmtgt, whereby the system stores the target enginerotational speed Netgt and the target generator field-coil currentIfmtgt, thus determined; and the corresponding generator torque Ta.

In Step S98, the system determines the target engine torque Tetgt basedupon the generator torque Ta obtained in Step S97. First, the systemcalculates the product of the generator torque Ta and the pulley ratioRp, whereby the load torque corresponding to that of the engine outputshaft is calculated. Furthermore, the system makes the sum of: the loadtorque thus obtained; the torque Ttcin transmitted to the torqueconverter, which is calculated based upon the present front-wheel speedand the target engine rotational speed Netgt; and the other term Texwhich is the sum of the driving torque of the other sub-members of theengine, the friction torque, and so forth, whereby the target enginetorque Tetgt is calculated.

In Step S99, the system stores the results thus obtained, in the memory.Specifically, the system stores: the target motor-field-coil currentIfmtgt; the target generated current Iatgt generated by the generator;the target generator field-coil current Ifatgt for obtaining the targetgenerated current; the target engine torque Tetgt; and the target enginerotational speed Netgt. As described above, the loop processing in StepsS93 through S99 is repeated n times, and accordingly, in Step S910, theflow returns to Step S93 until the counter i reaches n.

In Step S911, the system searches the memory storing the results thusobtained, for the target rotational speed Netgt closest to the presentengine rotational speed, and further selects: the targetelectric-motor-field-coil current Ifmtgt; the target generator generatedcurrent Iatgt; the target generator-field-coil current Ifatgt forachieving the target generated current; and the target engine torqueTetgt, corresponding to the selected target rotational speed Netgt,whereby the processing ends.

In Step S10 shown in FIG. 9, the system adjusts theelectronically-controlled throttle 1 a so as to control and increase theoutput of the engine 1 based upon the target engine torque Tetgt and thetarget engine rotational speed Netgt, thus obtained. Furthermore, inStep S11 shown in FIG. 9, the system controls the generator 2 and themotor 5 so as to increase the driving force of the electric-motor-drivenwheels 8 based upon the target electric-motor-field-coil current Ifmtgt,the target generator generated current Iatgt, and the targetgenerator-field-coil current Ifatat.

Thus, with the second control method according to the presentembodiment, in a case of slipping of the engine-driven wheels 9, thesystem increases the output of the engine 1 so as to increase thedriving force of the electric-motor-driven wheels 8, unlike theconventional one, thereby increasing the total driving force of theengine-driven wheels 9 and the electric-motor-driven wheels 8, andthereby improving the acceleration performance. In other words, with thesecond control method described above, while the system does notsuppress slipping of the engine-driven wheels 9, the increase of theeffective driving force of the electric-motor-driven wheels 9 is greaterthan the decrease of the effective driving force of the engine-drivenwheels 9, thereby improving acceleration performance.

That is to say, the second control method according to the presentembodiment improves the acceleration driving performance in a case ofslipping during acceleration as shown in FIG. 14. FIG. 14 shows theoperation of the hybrid-four-wheel-drive vehicle according to thepresent embodiment in a case wherein the vehicle climbs an icy uphillslope, wherein the second control method exhibits greater hill-climbingacceleration than with the first control method. FIG. 14A shows theoperation of the accelerator pedal by the user, wherein at the point intime A, the user starts stepping of the accelerator pedal, and maintainsthe stepping amount thereof from the point in time B. FIG. 14B shows thefront-wheel speed, and FIG. 14C shows the rear-wheel speed,corresponding to the duration shown in FIG. 14A. In the drawings, thesolid lines represent the operation of the hybrid-four-wheel-drivevehicle according to the second control method, and the broken linesrepresent the operations thereof according to the first control method.Note that with the first control method, in the event that the systemdetects slipping of the front wheels, the system reduces the output ofthe engine so as to suppress slipping thereof for increasing the drivingforce of the engine-driven front wheels, which is applied toconventional hybrid-four-wheel-drive vehicles including a so-called atraction control device.

As can be understood from comparison between the front-wheel-speed datarepresented by the solid line and the broken line shown in FIG. 14B,with the second control method, upon the user stepping the acceleratorpedal, the front-wheel speed increases at a constant rate. On the otherhand, with the first control method, the front-wheel speed does notincrease from the point in time A′ shown in FIG. 14D, at which thedifference in speed between the front and rear wheels exceeds apredetermined value, by the actions of the traction control device. Notethat the transaction control device controls so as to reduce thethrottle valve aperture TVO based upon the stepping amount of theaccelerator-pedal, and accordingly, the action of the traction controldevice can be confirmed by monitoring the action of the throttle valveaperture TVO. As shown in FIG. 14I, with the first control method, thethrottle valve aperture TVO is temporarily reduced during a period intime from the point A′. Subsequently, the throttle valve aperture iscontrolled independent of intention of the user (the stepping amount ofthe accelerator pedal).

FIG. 14E shows the acceleration data of the vehicle, wherein theacceleration of the vehicle controlled according to the second controlmethod represented by the solid line exhibits increased acceleration,despite slipping of the front wheels. Description will be made belowregarding the mechanism with reference to FIGS. 14F, 14G, and 14H, whichshow the change in the front-wheel driving force over time, the changein the rear-wheel driving force over time, and the change in the totaldriving force, respectively. As shown in FIG. 14F, with the secondcontrol method, the effective front-wheel driving force Tfweff isgradually reduced from the point in time of occurrence of front-wheelslipping. The reason is that the system controls the throttle so as toincrease the throttle valve aperture TVO for increasing the generatedelectric power generated by the generator as shown in FIG. 14I, leadingto reduced effective driving force as described with reference to FIG.8. On the other hand, in this case, the output of the electric motor isincreased so that the increase of the effective rear-wheel driving forceis greater than the decrease of the effective front-wheel driving forceas shown in FIG. 14G. As a result, the second control method effects theincreased effective driving force of the vehicle as compared with thefirst control method, as shown in FIG. 14H.

Next, description will be made below regarding switching means forswitching between the first control method and the second control methodin Step S5 shown in FIG. 9, by way of a specific arrangement exampleshown in FIGS. 15 and 16. FIG. 15 shows an arrangement wherein the userselects a desired control method from the first and second controlmethods through a switch. FIG. 16 shows an arrangement wherein thesystem automatically switches the control method corresponding todriving situations of the vehicle.

FIGS. 15A and 15B show a configuration of the switch 12 shown in FIG. 1,wherein FIG. 15A is a front view of the switch 12, and the FIG. 15B is aconceptual configuration diagram thereof. The switch 12 has aconfiguration wherein the user selects the control method by turning aknob 162. Specifically, upon the user setting the knob 162 to theposition A, the two-wheel-drive (2WD) mode, wherein the vehicle isdriven by the engine alone, is selected. On the other hand, upon theuser setting the knob 162 to the position B, or C, the four-wheel-drive(4WD) mode is selected. In particular, upon the user setting the knob162 to the position B, the system selects the first control method(MOT-priority mode), and upon the user setting the knob 162 to theposition C, the system selects the second control mode (TCS). Asdescribed above, with the present embodiment, the user can select adesired control method from the prepared control methods by onlyoperating the single switch 12, thereby facilitating the user to confirmthe presently-selected control method, as well as facilitating the userto select a desired control method. Furthermore, the switch 12 includesa spring 163 at the position C as shown in FIG. 15B, and has aconfiguration wherein upon the user turning the knob 162 to the positionC, the knob 162 is automatically returns to the knob position B after apredetermined period of time. The reason is that following controlprocessing according to the second control method thus selected whereinthe system gives priority to the increase of output of the electricmotor so as to increase the driving force of the rear wheels underconditions such as driving on an icy uphill slope, the systemautomatically switches the presently-selected control method back to thefirst control method wherein the system gives priority to suppression ofslipping of the front wheels during acceleration, thereby preventingdeterioration in the lifespan of the electric motor due to excessive usethereof, and thereby preventing deterioration in the driving performanceof the vehicle. Note that with such a configuration wherein thepresently-selected control method is automatically returned back to thefirst control method, the user cannot confirm whether the systempresently selects the first control method or the second control methodonly by checking the switch position. Accordingly, with the presentembodiment, the switch 12 further includes lamps 161 at the knobpositions A, B, and C, for indicating the presently-selected controlmethod, thereby notifying the user of the presently-selected controlmethod in a sure manner. Note that the type of the switch 12 used hereis not restricted to a rotary switch as shown in FIG. 15, rather, apush-button switch, a rocker switch, may be employed as the switch 12.

Next, description will be made regarding an arrangement wherein thesystem automatically makes switching between the first and secondcontrol methods corresponding to driving situations of the vehicle withreference to FIG. 16. In Step S101, the system computes and stores theeffective driving force Tfweff which is a part of the front-wheeldriving force and contributes acceleration of the vehicle. Thecomputation processing is consecutively performed, and the system storesthe history data for the past five seconds, for example. The effectivedriving force Tfweff is calculated based upon: the driving force of thefront wheels; the driving force of the rear wheels; the front-wheelacceleration obtained by differentiating the front-wheel speed; thespeed and the acceleration of the rear wheels; the mass of the vehicle;and the acceleration of the vehicle. Note that the acceleration of thevehicle may be obtained using an acceleration sensor, or may becalculated based upon the rear-wheel speed for simplification. Now,description will be made below by way of the aforementioned simplemethod. The effective front-wheel driving force Tfweff matches thedifference between the front-wheel driving force Tfw and the torquewhich contributes acceleration of the front wheels. Here, the torquewhich contributes acceleration of the front wheels is calculated bymaking the product of the moment of inertia of the front wheels and thedriving system thereof, Jfw, and the angular acceleration Nrw. Note thatthe angular acceleration is calculated by differentiating the wheelspeed detected by the speed detecting means. The computation processingdescribed above is represented by the following Expression (3).Tfweff=Tfw−Jfw·Nrw  (3).

In Step S102, the system computes and stores the effective rear-wheeldriving force Trweff based upon: the rear-wheel driving force, Trw; themoment of inertia of the driving system of the rear wheels, Jrw; and theangular acceleration thereof, Nfw, in the same way as in Step S101.

In Step S103, the system calculates the vehicle-acceleration requestvalue based upon the stepping amount of the accelerator pedal, andfurther calculates the target total effective driving force, Tvclefftgt,for the front and rear wheels for achieving the acceleration request.The target total effective driving force Tvclefftgt is obtained bymaking the sum of: the effective front-wheel driving force Tfweff; theeffective rear-wheel driving force Trweff; and the product of the changein the stepping amount of the accelerator pedal ΔTh, which has beencomputed and indicates the acceleration request, and a constant C2.

In Step S104, the system calculates the maximum effective front-wheeldriving force Tfweffmax based upon the history data of the front-wheelslippage, the history data of the effective driving force thereof, andthe history data of the driving force thereof. Note that in a case ofthe maximum front-wheel slippage stored in the history data of 10% orless, the system obtains the maximum effective driving force byextrapolating the history data of the effective driving force in theslippage range of 10 to 20%.

In Step S105, the system performs the computation processing forcalculating the maximum effective rear-wheel driving force Trweffmax inthe same way as in Step S104. In this case, with an arrangement whereinthe slippage can be calculated using the data from the accelerationsensor or the like, the rear-wheel slippage can be calculated in thesame way as with the front-wheel slippage. However, with the presentarrangement employing the aforementioned simple method wherein thevehicle speed is calculated based upon the rear-wheel speed, therear-wheel slippage cannot be calculated. Accordingly, in this case, themaximum effective rear-wheel driving force is set to a predeterminedvalue beforehand.

In Step S106, the system calculates the difference between the presenteffective front-wheel driving force Tfweff and the maximum effectivedriving force Tfweffmax, whereby the front-wheel driving-force marginΔTfweff up to the maximum effective driving force is obtained. In StepS107, the system calculates the rear-wheel driving force margin ΔTrweffin the same way.

In Steps S108 through S112, the system computes reduction of theeffective front-wheel driving force and the increase of the effectiverear-wheel driving force in a case of control processing according tothe second control method wherein the system increases the enginerotations and engine torque so as to increase the output of the electricmotor for achieving the target acceleration of the vehicle, whereby thetotal effective driving force Tvcleff2 is obtained. On the other hand,in Steps S113 through S115, the system calculates the total effectivedriving force Tvcleff1 for the front and rear wheels in a case ofcontrol processing according to the first control method wherein thesystem reduces the engine output so as to increase the effectivefront-wheel driving force.

In Step S108, the system calculates the target electric-motor torqueTmtgt2. The target electric-motor torque Tmtgt2 is calculated asfollows. That is to say, the maximum effective front-wheel driving forceTfweffmax calculated in Step S104 is multiplied by a constant C3.Subsequently, this value thus obtained is subtracted from the targettotal effective driving force Tvclefftgt for the front and rear wheels,calculated in Step S103, on the assumption that slipping of the rearwheels does not occur, whereby the target rear-wheel driving forceTrwtgt is obtained. Subsequently, the system calculates the targetelectric-motor torque Tmtgt2 based upon the target rear-wheel drivingforce Trwtgt, giving consideration to the reduction ratio of thedifferential mechanism. While strictly, the effective front-wheeldriving force changes corresponding to the slippage of the front wheels,in practice, an arrangement may be made wherein the effectivefront-wheel driving force is determined to be the maximum effectivefront-wheel driving force multiplied by the constant C3 which representsan integer less than 1, for reducing calculation time. Here, theempirical constant C of 0.8 is employed.

In Step S109, the system computes the target engine rotational speedNetgt and the target engine torque Tetgt, required for outputting thetarget electric-motor torque Tmtgt. The computation processing isperformed in the same way as in Steps S91 through S911.

In Step S110, the system calculates the front-wheel driving force Tfw2based upon the target engine rotational speed Netgt, the target enginetorque Tetgt, and the properties of the torque converter. In Step S111,the system calculates the effective front-wheel driving force Tfweff2based upon the front-wheel driving force Tfw1.

In Step S112, the system makes the sum of the effective front-wheeldriving force Tfweff2 and the effective rear-wheel driving forceTrweff2, whereby the total effective driving force Tvcleff2 for thefront and rear wheels is obtained.

In Step S113, the system calculates the engine rotational speed Ne andthe engine torque Te for achieving the maximum effective front-wheeldriving force Tfweffmax, giving consideration to a case wherein thesystem selects the first control method. Note that the computation ismade based upon the front-wheel rotational speed and the properties ofthe torque converter.

In Step S114, the system calculates the electric-motor torque Tm basedupon the generated voltage and current from the generator, calculatedbased upon the target rotational speed Netgt and the target enginetorque Tetgt, which have been obtained in Step S113, and furthercalculates the rear-wheel driving force and the effective rear-wheeldriving force Trweff1.

In Step S115, the system makes the sum of the effective rear-wheeldriving force Trweff1 and the target maximum effective front-wheeldriving force Tfweffmax, obtained in Steps S112 through S114, wherebythe effective driving force Tvcleff1 according to the first controlmethod is determined.

In Step S116, the system makes comparison between the total effectivedriving force Tvcleff1 obtained in Step S114 and the total effectivedriving force Tvcleff2 obtained in Step S112. In the event thatdetermination has been made that Tvcleff1 is greater than Tvcleff2, theflow proceeds to Step S117, wherein the system sets the flag CntrlFlagto “1” for selecting the first control method. On the other hand, in theevent that determination has been made that Tvcleff2 is greater thanTvcleff1, the flow proceeds to Step S118, wherein the system sets theflag CntrlFlag to “2” for selecting the second control method.

That is to say, the automatic switching means shown in FIG. 16comprises: effective driving force computation means (S102, S102) forcomputing the effective engine-driven-wheel driving force and theeffective electric-motor-driven-wheel driving force; accelerationdriving force computation means for computing the acceleration drivingforce C2·ΔTh for all the driven wheels corresponding to the accelerationrequest value; target effective driving force computation means (S103)for computing the target effective driving force by making the sum ofthe present effective engine-driven-wheel driving force, the presentelectric-motor-driven-wheel driving force, computed by the effectivedriving force computation means, and the acceleration driving force;effective driving force history computation means (S101, S102) forcreating the history data of the effective engine-driven-wheel drivingforce and the effective electric-motor-driven-wheel driving forcecorresponding to the last slippage data for a predetermined past period;and maximum effective driving force computation means (S104, S105) forcalculating the maximum effective engine-driven-wheel driving force andthe maximum effective electric-motor-driven-wheel driving force basedupon the history data. Thus, the system obtains the relation between theslippage and the effective driving force shown in FIG. 5, determined bythe relation between the road on which the vehicle is presently drivenand the properties of the driven wheels.

Subsequently, in Steps S108 through S112, the system estimates the totaleffective driving force in a case of selecting the second controlmethod, based upon the relation between the present slippage and theeffective driving force calculated in Steps S101 through S105. On theother hand, in Steps S113 through S115, the system estimates the totaleffective driving force in a case of selecting the first control methodin the same way. Subsequently, in Step S116, the system makes comparisonbetween the estimated total effective driving force according to thefirst control method and the estimated total effective driving forceaccording to the second control method, and the control methodcorresponding to the greater total effective driving force is selected.

In other words, the switching means shown in FIG. 16 comprises: theeffective driving force computation means (S101, S102) for computing thepresent effective engine-driven-wheel driving force and the presenteffective electric-motor-driven-wheel driving force; the accelerationdriving force computation means (S63 in FIG. 10) for computing theacceleration driving force for achieving the acceleration request value;the target effective driving force computation means (S103) forcomputing the target effective driving force by making the sum of thepresent effective engine-driven-wheel driving force, the presentelectric-motor-driven-wheel driving force, computed by the effectivedriving force computation means (S101, S102), and the accelerationdriving force; the effective driving force history computation means(S101, S102) for creating the history data of the effectiveengine-driven-wheel driving force and the effectiveelectric-motor-driven-wheel driving force corresponding to the lastslippage data for a predetermined past period; and the maximum effectivedriving force computation means (S104, S105) for calculating the maximumeffective engine-driven-wheel driving force and the maximum effectiveelectric-motor-driven-wheel driving force based upon the history data.

Furthermore, the switching means shown in FIG. 16 further comprises:first target engine-output computation means (S113) for calculating thefirst target engine output corresponding to the maximum effectiveengine-driven driving force obtained by the maximum effective drivingforce computation means (S104, S105); effective driving forcecomputation means (S114) for computing the effective engine-driven-wheeldriving force and the effective electric-motor-driven-wheel drivingforce corresponding to the first target engine output based upon theaforementioned history data; and first total effective driving forcecalculating means (S115) for calculating the target total effectivedriving force for the engine-driven wheels and the electric-motor-drivenwheels according to the first control method.

Furthermore, the switching means shown in FIG. 16 further comprises:target electric-motor-driven-wheel driving force computation means(S108) for calculating the target driving force of theelectric-motor-driven wheels by subtracting the maximum effectiveengine-driven-wheel driving force multiplied by a predeterminedreduction coefficient from the target effective driving force obtainedby the target effective driving force computation means (S103); targetelectric-motor torque computation means (S108) for calculating thetarget electric-motor torque based upon the target driving force of theelectric-motor-driven wheels; second target engine output computationmeans (S109) for calculating the target engine output required forachieving the target electric-motor output; target effectiveengine-driven-wheel driving force computation means (Sl10, S111) forcalculating the target effective driving force of the engine-drivenwheels corresponding to the aforementioned target engine output; andsecond total effective driving force computation means (S112) forcalculating the target total effective driving force according to thesecond control method by making the sum of the target effectiveelectric-motor-driven-wheel driving force and the target effectiveengine-driven-wheel driving force.

Then, with the switching means shown in FIG. 16, in Step S116, thesystem makes comparison between the target total effective driving forceaccording to the first and second control methods, and selects thecontrol method corresponding to the greater target total effectivedriving force. With the present embodiment, the controller 4 includesthe processing means shown in Steps S101 through S118, thereby enablingautomatic switching of the control method wherein the system predictsdriving of the vehicle beforehand, and automatically switches thecontrol method so as to achieve great acceleration of the vehicle.

With the present embodiment, the maximum effective driving forcecomputation means (S105) calculate the maximum effective driving forceof the electric-motor-driven wheels corresponding to the last slippagedata for a predetermined past period, obtained by the effective drivingforce history computation means (S101, S102) shown in FIG. 16, and thus,the system determines the target electric-motor torque Tetgt in a rangeof under the maximum effective electric-motor-driven-wheel drivingforce, in Step S9 in FIG. 9.

As described above, with the present embodiment, the system switches thedriving state of the vehicle according to the intention of the user.However, in some cases, the second control method, wherein the systemgives priority to the output of the electric motor, causes accelerationslipping of the front wheels, leading to a problem that steering of thefront wheels generates small lateral force of the wheels, i.e., leadingto a problem of so-called under-steering. Accordingly, an arrangementmay be made wherein in the event that the steering sensor 3 serving asthe steering amount detecting means shown in FIG. 1 detects the steeringamount greater than a predetermined value during control processingaccording to the second control method, the system switches the selectedcontrol method to the first control method. With such a configuration,at the time of the user steering the vehicle so as to turn a cornerwhile making acceleration, the vehicle generates yaw moment morequickly, thereby improving turning-round performance of the vehicle.

On the other hand, at the time of driving of the vehicle at a low speed,in many cases, the great yaw moment is not required. Accordingly, anarrangement may be made wherein the control method is switched givingconsideration to the speed detected by the wheel-sensors mounted to therear wheels. That is to say, an arrangement may be made wherein thesystem overrides to switch from the second control method to the firstcontrol method according to detection of steering in a case of thepresent vehicle speed exceeding the first vehicle-speed threshold (e.g.,8 km/h). With such a configuration, the vehicle maintains the greatrear-wheel driving force at a low speed even in a case of the usersteering the vehicle, thereby maintaining acceleration performance ofthe vehicle.

Furthermore, an arrangement may be made wherein in the event that thevehicle speed detected by the wheel-sensors mounted to the rear wheelsexceeds a predetermined second vehicle-speed threshold, the systemswitches the selected control mode from the second control method to thefirst control mode. This improves fuel efficiency.

On the other hand, in general, the greater the rotational speed of theelectric motor is, not only the smaller the torque thereof is, but alsothe efficiency thereof drops above a certain rotational speed. With thesecond control method, the system controls the engine output such thatthe output of the electric motor reaches the target value, andaccordingly, driving of the electric motor with poor efficiency requiresexcessive engine output, leading to poor fuel efficiency. Accordingly,an arrangement may be made wherein the system detects the rotationalspeed of the electric motor using an unshown electric-motor rotationalspeed sensor, and in the event that the detected electric-motorrotational speed is equal to or greater than a predetermined value, thesystem switches the control method from the second control method to thefirst control method. Or, an arrangement may be made wherein in theevent that the wheel speed detected by the wheel-speed detecting meansis equal to or greater than a predetermined value (e.g., 30 km/h), thesystem switches the control method from the second control method to thefirst control method. Note that with such a configuration wherein thevehicle speed is used for determination of control-mode switching, whileeither wheel speed may be used as the vehicle speed, the speed of therear wheels which are driven by the electric motor is more preferablyused as the vehicle speed. With an arrangement according to the presentembodiment including the differential gear 6, the system preferably usesthe average of the left and right rear-wheel speeds so as to cancel thedifference in wheel speed between the left and right wheels due to thedifferential gear 6, thereby further improving precision of the detectedvehicle speed.

Note that while description has been made in the aforementionedembodiment regarding an arrangement wherein the controller 4 is includedin a single casing, it is needless to say that an arrangement may bemade wherein the engine control means and the electric-motor controlmeans are included in separate casings, for example.

1. A control device for controlling a hybrid-four-wheel-driven vehiclewherein one of the front-wheel pair and the rear-wheel pair is anengine-driven-wheel pair which is driven by an engine, and the otherpair is an electric-motor-driven-wheel pair which is driven by anelectric motor connected to a generator driven by said engine, saidcontrol device comprising: slipping detecting means for detectingslipping of said engine-driven wheels; and output control means forincreasing the output of said engine corresponding to the increasedoutput of said electric motor, wherein said slipping detecting meansdetects slipping in the event that the speed of said engine-drivenwheels exceeds the driving speed of the vehicle.
 2. A control device forcontrolling a hybrid-four-wheel-driven vehicle wherein one of thefront-wheel pair and the rear-wheel pair is an engine-driven-wheel pairwhich is driven by an engine, and the other pair is anelectric-motor-driven-wheel pair which is driven by an electric motorconnected to a generator driven by said engine, said control devicecomprising: slipping detecting means for detecting slipping of saidengine-driven wheels; and output control means for increasing the outputof said engine corresponding to the increased output of said electricmotor, wherein said slipping detecting means detects slipping in theevent that the slippage, which is the difference in rotational speedbetween said engine-driven wheels and said electric-motor-driven wheels,divided by the driving speed of the vehicle, is equal to or greater thana predetermined value.
 3. A control device for controlling ahybrid-four-wheel-driven vehicle wherein one of the front-wheel pair andthe rear-wheel pair is an engine-driven-wheel pair which is driven by anengine, and the other pair is an electric-motor-driven-wheel pair whichis driven by an electric motor connected to a generator driven by saidengine, said control device comprising: slipping detecting means fordetecting slipping of said engine-driven wheels; and output controlmeans for increasing the output of said engine corresponding to theincreased output of said electric motor, wherein said output controlmeans further comprises: means for computing the present electric-motoroutput based upon an input current and a field-coil current of saidelectric motor; means for computing target acceleration driving forcecorresponding to an input acceleration request; means for obtainingtarget electric-motor output based upon said present electric-motoroutput and said target acceleration driving force; means for obtainingtarget engine output required for achieving said target electric-motoroutput; and means for controlling output of said engine and output ofsaid electric motor according to said target engine output and saidtarget electric-motor output.
 4. A control device for controlling ahybrid-four-wheel-driven vehicle wherein one of the front-wheel pair andthe rear-wheel pair is an engine-driven-wheel pair which is driven by anengine, and the other pair is an electric-motor-driven-wheel pair whichis driven by an electric motor connected to a generator driven by saidengine, said control device comprising: slipping detecting means fordetecting slipping of said engine-driven wheels; and output controlmeans for increasing the output of said engine corresponding to theincreased output of said electric motor, wherein said output controlmeans further comprises: effective-driving-force history computationmeans for obtaining history data of the effective driving force of saidelectric-motor driving wheels corresponding to the last slippage datafor a predetermined past period; and maximum-effective-driving-forcecomputation means for computing the maximum value of the effectivedriving force of said electric-motor-driven wheels based upon saidhistory data, wherein the output of said electric motor is increased ina range determined by the maximum value of the effective driving forceof said electric-motor-driven wheels.